Alex, I am not sure if the wahoo has been in the ground for more than just this season, but I am going to assume it has since you are contemplating a re-do.
This first post offered by dyzzypyxxy is great advice. While having a mulched area under the tree to prevent root competition from other plants will always help, there is one thing with which I take exception: you don't need a 20 ft. diameter circle surrounding a wahoo barely 6 ft. high. A 6 ft. diameter is adequate. Furthermore, our native Minnesota wahoos grow in the forest edges and understory of other trees. They are well adapted to root competition from every other plant and tree in the woods. Grass grows right up to the stems of my wahoos, and although they would likely do better with a mulch, they are just fine.
I can't imagine that it would need more water, with all the rain we have been getting in Mpls/St. Paul. If the hosta and ferns under the tree are not wilting, the tree does not need water.
An oak tree will grow 60ft high and 30ft wide in a forest, and 30ft high and 60ft wide in an open pasture. Tall and narrow in a forest and short and wide in the open land is a natural adaption, and this is what is happening with your wahoo. You never said how wide your tree is, but one might encourage vertical growth by trimming some off the side.